The invention relates to rolling bearings adapted for use in vacuum and high-temperature environments, and more particularly to an improvement of a lubricating film.
For rolling bearings used in the aforementioned environments where lubricating oils and greases cannot be applied, it is well known that at least one of the surface of a race member(s) defining a race, the surface of a rolling element, and the surface of the pocket of a retainer is coated with a lubricating film made of soft metal.
It is also well known that a material of which the lubricating film is made is silver or lead. For example, a lubricating film applied to a rolling bearing for X-ray tubes is subjected to such high ambient temperatures as 400.degree. to 500.degree. C., and this causes silver to lubricate in the solid state and lead in the molten state. Therefore, it can be said that lead is superior to silver for achieving better lubrication and noiseless running. Hence, lead is suitable as a material of which the aforementioned lubricating film applied to the rolling bearing for X-ray tubes is made.
If used as a material of which a lubricating film is made with steel being an object to be lubricated thereby, lead is relatively weak in adhesive strength in such a condition as to coat the steel surface. Therefore, an intermediate layer made of Sn or Pt is generally interposed between lead and steel in order to improve adhesive strength of lead. There is a strong tendency to use Sn because Pt is expensive and has a high melting point.
By the way, the lubricating film made of the aforementioned alloy has poor wettability with respect to steel in the molten state, making itself hard to stay in film form on the race surfaces of the race members, the surface of the rolling element, and the surface of the pocket of the retainer. Once such lubricating film has been removed from where the lubricating film is coated as a result of use of the bearing for a long period of time, it becomes difficult to cause a lubricant to be present on the surface of contact between the rolling element and the race surface of each of the inner and outer race members, as well as on the surface of contact between the rolling element and the surface of the pocket of the retainer. As a result, lubrication among the bearing components is decreased, which in turn causes the bearing to stick by contact between steel portions. Consequently, this brings about the problem of life.